I wouldn't invest any more money in this notebook. The CPU performance of a Lenovo T420 is simply no longer up to date. I would therefore invest the money in a new device. The best CPU, which was installed in the T420 at the time, achieved just under 2500 in the Passmark Peformance Test. Today, these values are already surpassed by an Intel Celeron N4125 processor, which can be found in almost all notebooks from 250 francs upwards.

Here is a Passmark representation of the computing performance (fastest CPU in the T420 vs Celeron vs Pentium Gold 11th Gen):

https://www.cpubenchmark.net/compare...

It's a bit of a shame, but I'm afraid a new notebook will do a lot more good. The cheapest Windows 10/11 compatible notebook would be the HP 250 G8. It has 8GB RAM, Windows 10 and a new 11th Gen Pentium Gold 7505 processor installed that is more than twice as fast as the best T420 machine. And the crazy thing is that the HP 250 G8 is already available for 379 francs, including a Full HD display with 1920 x 1080 pixels on 15.6 inches.

HP 250 G8 (15.60 ", Intel Pentium Gold 7505, 8 GB, 256 GB)