How to merge APFS volumes because of low perfomance
The partitions should have nothing to do with performance issues, I have the same computer and three partitions on the disk as well. I would run the hardware test that is mentioned above and you can also try resetting the NVRAM (https://support.apple.com/en-us/HT204063) and you can try resetting the SMC as well (https://support.apple.com/en-us/HT201295). Apple recommends resetting the NVRAM/PRAM first and then resetting the SMC. I normally find resetting the SMC to do the trick though. Also, don’t worry, neither of these will hurt your computer if they do not work.