优化完成，速度可压至<1ms。

using System.Text;

using System;

namespace VideoSlice.Cli;

    /// <returns>时间表达式</returns>

    public static string ToString(TimeSpan timeSpan)

    {

        var days = timeSpan.Days;

        var hours = timeSpan.Hours;

        var minutes = timeSpan.Minutes;

        var seconds = timeSpan.Seconds;

        var milliseconds = timeSpan.Milliseconds;

        var microseconds = timeSpan.Microseconds;

        var len =

            GetStringLengthFast(timeSpan.Days) + 1

            + GetStringLengthFast(timeSpan.Hours) + 1

            + GetStringLengthFast(timeSpan.Minutes) + 1

            + GetStringLengthFast(timeSpan.Seconds) + 1

            + GetStringLengthFast(timeSpan.Milliseconds) + 2

            + GetStringLengthFast(timeSpan.Microseconds) + 2

            + 4;

        var sb = new StringBuilder(len);

        Append(timeSpan.Days, "d", sb);

        Append(timeSpan.Hours, "h", sb);

        Append(timeSpan.Minutes, "m", sb);

        Append(timeSpan.Seconds, "s", sb);

        Append(timeSpan.Milliseconds, "ms", sb);

        Append(timeSpan.Microseconds, "us", sb);

        return sb.ToString();

            GetStringLengthFast(days) + 1

            + GetStringLengthFast(hours) + 1

            + GetStringLengthFast(minutes) + 1

            + GetStringLengthFast(seconds) + 1;

        if (milliseconds != 0)

            len += GetStringLengthFast(milliseconds) + 2;

        if (microseconds != 0)

            len += GetStringLengthFast(microseconds) + 2;

        return string.Create(len, timeSpan, static (span, ts) =>

        {

            var pos = span.Length;

            if (ts.Microseconds != 0)

                AppendFromEnd(span, ref pos, ts.Microseconds, "us");

            if (ts.Milliseconds != 0)

                AppendFromEnd(span, ref pos, ts.Milliseconds, "ms");

            AppendFromEnd(span, ref pos, ts.Seconds, "s");

            AppendFromEnd(span, ref pos, ts.Minutes, "m");

            AppendFromEnd(span, ref pos, ts.Hours, "h");

            AppendFromEnd(span, ref pos, ts.Days, "d");

        });

    }

    /// <summary>

    /// <returns></returns>

    public static TimeSpan ToTimeSpan(string str)

    {

        var timespan = TimeSpan.Zero;

        timespan = timespan.Add(TimeSpan.FromDays(Parse(str, "d")));

        timespan = timespan.Add(TimeSpan.FromHours(Parse(str, "h")));

        timespan = timespan.Add(TimeSpan.FromMinutes(Parse(str, "m")));

        timespan = timespan.Add(TimeSpan.FromSeconds(Parse(str, "s")));

        timespan = timespan.Add(TimeSpan.FromMilliseconds(Parse(str, "ms")));

        timespan = timespan.Add(TimeSpan.FromMicroseconds(Parse(str, "us")));

        var days = 0;

        var hours = 0;

        var minutes = 0;

        var seconds = 0;

        var milliseconds = 0;

        var microseconds = 0;

        return timespan;

    }

        var i = 0;

        var len = str.Length;

    private static void Append(int value, string unit, StringBuilder sb)

        while (i < len)

        {

        sb.Append($"{value}{unit}");

    }

    public static int Parse(string s, string unit)

    {

        var sliced = s.Split(unit);

            if (str[i] == '-')

                throw new InvalidOperationException("值不能为负数。");

        var str = Near(sliced[0]);

            if (!char.IsAsciiDigit(str[i]))

                throw new InvalidOperationException($"'{str[i]}' 不是有效的字符。");

        if (int.TryParse(str, out var value))

            var value = 0L;

            var numberStart = i;

            do

            {

            return value < 0 ? throw new InvalidOperationException("值不能为负数。") : value;

        }

                value = value * 10 + (str[i] - '0');

                if (value > int.MaxValue)

                    throw new InvalidOperationException(

                        $"'{str.Substring(numberStart, i - numberStart + 1)}' 不是一个有效的32位整数。");

                i++;

            } while (i < len && char.IsAsciiDigit(str[i]));

        throw new InvalidOperationException($"'{sliced[0]}' 不是一个有效的32位整数。");

    }

            var unitStart = i;

            while (i < len && !char.IsAsciiDigit(str[i]) && str[i] != '-')

                i++;

    private static string Near(string s)

    {

        var arr = new char[s.Length];

        var count = 0;

            var unitLen = i - unitStart;

        for (var i = s.Length - 1; i >= 0; i--)

            switch (unitLen)

            {

            if (char.IsAsciiDigit(s[i]))

                case 1:

                    switch (str[unitStart])

                    {

                arr[i] = s[i];

                count++;

                        case 'd': days = (int)value; break;

                        case 'h': hours = (int)value; break;

                        case 'm': minutes = (int)value; break;

                        case 's': seconds = (int)value; break;

                        default:

                            throw new InvalidOperationException($"'{str[unitStart]}' 不是有效的单位。");

                    }

                    break;

                case 2:

                    if (str[unitStart] == 'm' && str[unitStart + 1] == 's')

                        milliseconds = (int)value;

                    else if (str[unitStart] == 'u' && str[unitStart + 1] == 's')

                        microseconds = (int)value;

                    else

                        throw new InvalidOperationException($"'{str.Substring(unitStart, 2)}' 不是有效的单位。");

                    break;

                default:

                    throw new InvalidOperationException($"'{str.Substring(unitStart, unitLen)}' 不是有效的单位。");

            }

        }

        if (count == 0)

            throw new InvalidOperationException("只包含非数字字符串。");

        return new TimeSpan(days, hours, minutes, seconds, milliseconds, microseconds);

    }

    private static void AppendFromEnd(Span<char> span, ref int pos, int value, string unit)

    {

        for (var j = unit.Length - 1; j >= 0; j--)

            span[--pos] = unit[j];

        var negative = value < 0;

        var v = negative ? -(long)value : value;

        do

        {

            span[--pos] = (char)('0' + v % 10);

            v /= 10;

        } while (v != 0);

        return new string(arr, arr.Length - count, count);

        if (negative)

            span[--pos] = '-';

    }

    private static int GetStringLengthFast(int value)