I would venture a guess that the network speed on that RPi is being limited by the NIC not having full hardware offloading and is bottlenecking on the weak CPU. This would make sense why the speed slows down a bit when Kodi is running. You could try different overclock settings (remember we removed these to stop the freezing) and perhaps you could gain a bit from that while staying stable. It could be that your power supply had gotten a bit weak and couldn’t sustain that overclock your running so if you have another you could try swapping it out and try your old settings again. Upgrading hardware to something more modern would be the most ideal situation though.
I have been thinking the same thoughts regarding the overclocking and the power supply.
Having tried all the power supplies I currently have, and finding that none of the others seem to supply enough power I guess trying less aggressive overclocking settings using the current one is a job to try tomorrow.
There may be a balance that works in the short term between overclocking and not overloading the power supply.