If your old iPhone feels slow and sluggish, it might be because the battery is too old. Replacing it will improve the performance of your iPhone instantly.
A post on Reddit said that iOS will purposely slow down a phone when the battery shows substantial wear. I have a 3 year old iPhone 6+. I measured the battery condition using coconutBattery from my Mac. The battery was down to 75% of design capacity after ~750 cycles. I then ran Geekbench for iOS. The benchmarks were about 1000 for single-core performance, and 1800 for multi-core. They should be 1460/2459 respectively. Finally, I installed CPU DasherX on my phone. Down towards the bottom it reports the CPU frequency. It said 839MHz for my phone, but it should be 1400 MHz.
I went to a local place to replace the battery for $50. Now all my numbers are as they should be. The phone works perfectly well. I almost bought a new phone because this one had turned sluggish. I just saved myself $800 for a phone I didn’t need.