Ghost touch and lag problem

Ghost touch problems are usually caused by a defective digitizer on the screen, and replacing the screen is the normal cure for that problem.

As far as the lag, that could be caused by the phone having to constantly handle bogus touchscreen events and not having time left over for normal operations, so I'd suggest replacing the screen first then reevaluating whether that's also fixed the lag problem.

There aren't a lot of guides available for your phone, but I did locate a YouTube video that shows how to replace the screen. It's definitely not a professional quality guide, but looks like it has all the information needed to do the job.

Realme C25y Lcd Replacement

The screens are available quite inexpensively at sites like Amazon, eBay and AliExpress, among others. Here's one of the first ones that came up.

For RealMe C25Y LCD Screen + Touch Digitizer Display 1560*720 RMX3265, RMX3268 For RealMe C25Y LCD Screen Display - AliExpress

Note that the screen is offered with and without the frame attached. Normally it's recommended to go with the one that comes with the LCD already mounted on the frame as there are less parts to have to reattach when you're done and it's likely to be better sealed than what you can do manually. However, you'd be on your own with that repair, as the only video I found shows it being done on a display without the frame, so it's up to you as to which way to go.

Good luck and let us know how it goes! You can always take lots of pictures and use those to author your own guide to that repair for the benefit of others who come after you. It's fun, easy and you'll earn the appreciation of the community.

Create a new Guide - iFixit