Sending emails appears to slow down gradually

Has anybody also noticed that sending emails tends to slow down gradually ?

I tweaked config combined with crons to send 1500 emails per hour.

To do that I call processqueue every 10 minutes and have it send 250 emails.
And I started with leaving 1 second between each email.

That worked well for hours until I noticed it needed more time than 10 minutes to get the 250 emails out.
So I reduced the time between emails again to 0.5

Hours later I noticed it needed more than 10 minutes again, so I reduced it to 0.2
Again this fixed it but hours later (now as I’m writing this) I notice 10 minutes is not enough to get 250 emails out.

I wonder why that is ?
I have domain throttle on with a delay of 30 seconds.
But I assume the script simply skips such emails and jumps to the next record and so on ? (doesn’t wait)
And when all is done, starts again from the start and goes through all (unsent) records again ?

PS. I’m not yet at the latter state. I can totally understand that when the list is cycled a second time with many @gmail @hotmail and other such large domain emails, that sending slows down dramatically. This is not that scenario yet.

I wonder what could cause the gradual slowing down ?
Is it memory use building up ? Keeping track of things ?
Or is this just coincidence and I should go look for other reasons why the server might be busier now than before?

The domain throttle will alter your sending rate. I have seen that it slows it down.

That’s most likely what is going on.