While the MIDI bandwidth (3125 bytes/s) is much lower than that of USB (about 1 MB/s for USB 1.x), the USB protocol adds latencies of at least one millisecond for most packets, so for an application like MIDI where very few bytes are transmitted, the USB overhead indeed makes it slower.

However, even USB MIDI does not introduce noticeable delays in most circumstances. Nowadays, the largest problem is that software synthesizers, which are implemented on a general-purpose CPU an in a multitasking OS, cannot generate sounds with as low a latency as hardware synthesizers with a dedicated DSP.

The latency of MIDI is proportional to the length of the messages. So, if you play a 3-note chord, for example, it will take a time of, at least, 1,3 milisecond between the first note to the third note to be received at the other side of the cable. If the keyboard does not implement running status, the time required will be greater. And if the keyboard implements aftertouch, the delay will be even greater. But this will not be a problem on most cases.
As Clemens said, the main cause of latency is due to the software processing, i.e, audio drivers and, eventually, the proper response of the software synths.

To accurately answer your question, you would need to tell us specifically what you are using.

What OS?
What MIDI connections?
What DAW and software?

Unfortunately there are many timing factors for the computer OS because it is a generalized computing devices, not a musical instrument. Perhaps some of these articles may help.

optimising-latency-pc-audio-interface

MIDI Latency

I too have been in MIDI for a very long time ( over 30 years ). I build most the PCs I use from scratch. I too started off on very slow CPUs by today's standards. The older slower CPUs ran circles around today's dual and quad core monsters in terms of the lack of latency. I am a Songwriter/Composer. I have relied on MIDI to capture what I play at the keyboard. Latency severely hampers that process.

The operating systems since Windows XP have been slowly killing off the older card based interfaces and has been replacing them with more and more USB solutions. The USB solutions cause a constant running gun battle. This to me is backwards.

MIDI's beauty in the beginning was it's low overhead. What happened?
How do we fix this?

As the last poster above hints, a substantial part of the problem is WinDoze. W is a multi process system, that is usually doing other things at the same time, and will be constantly distracted, and there will be delays, and there's nothing you can do about it. Like is said above, it's running battle. All windows software is constantly being 'improved' with extra bells and whisttles. Every improvement takes back some, most or MORE of the gains provided by faster machines/CPUs etc.

I admit it, I'm an old-time DOS man. I use WinDoze, in effect, under duress. I use XP because the text interface/Command prompt is more reliably DOS than anything later.

However, ALL my midi stuff is on an old DOS-only Pentium 75 sitting next to the XP machine, all my midi modules etc are attached to that, the Roland sound card is in there (more modern macnines don't even have an ISA slot for it). Midi works FINE, never any hint of any speed problems. What's 'latency' (rhetorical question !).. Midi isn't slower, it's the newer machines it's being run on that are too busy trying to do other things at the same time.

Yes, I understand, you can do some neat things via Windows. But be prepared to pay the cost, and accept it.

Geoff