I am seeing very strange behavior and looking for some insight. The configuration is simple:

1. ISP modem connects to MikroTik RG750v3 port 1.
2. Computer 1 connects to a dumb switch, which is then patched to MikroTik port 2.
3. Computer 2 connects directly to MikroTik port 3 (for testing purposes.)

The ISP feed is supposed to be 1Gbps.

Here is what I am seeing:

a. If I do an Internet speed test from Computer 1, I generally get around 150Mbps
b. If I do an Internet speed test from Computer 2, I generally get around 500Mbps.
c. If I do an Internet speed test from Computer 1 within a few seconds of doing a speed test from Computer 2, then the speed bums up to around 500Mbps for around a minute or so. After that, the speed drops back down to 150Mbps.

I know this sounds strange, but I have tested it dozens of times, and the results are very consistent.

What are possible reasons that the speed on Computer 1 would increase when testing immediately after running a speed test on Computer 2? (Both computers are running Windows 10.)

I'd much appreciate any thoughts on this.