This 1951, 2.5 tonne beast has just been rebooted after restoration.
Restored the oldest digital computer still running | Tech News Pedia
It's difficult to compare to present day machines as they are so far removed. However, it is decimal rather than binary and originally had 20 x 8-digit dekatron counters which is roughly 160 bytes of RAM (later upgraded to 40 x 8-digit dekatron). Input via ticker tape and took up to 10 seconds to multiply two decimal numbers.
Harwell computer - Wikipedia, the free encyclopedia
You can view it at the National Museum of Computing, Bletchley Park