My LG TV takes 20 minutes to turn on.

Hello, what model?
How old is this TV? (approximately) If it is not recent
If on the contrary it is still under warranty, do not touch anything and contact customer service
It could be similar to a loose connection at the button or temperamental capacitors, but it's hard to say without being in the trade.

7 years is the decisive age for TVs
if everything goes well, the TV can last up to 10 years or more
but this is the age when serious breakdowns begin and there is no more warranty, even if you had the extension to 5 years.
of course, repairing it is not professionally feasible
(you would have to find a repairman who still knows how to do it, and at what price)
there is the option of waiting for it to finally give out like that, or investing in a new TV
prices for sizes up to 32 inches (diagonal of 82 cm) have significantly dropped
beyond that, it’s more expensive, but the recommended viewing distance we knew in the past is not entirely true anymore
I just bought a Samsung 4K in 50" or 125 cm diagonal, viewed from about 3 m away from the couch the picture is superb, nothing compared to my old Philips Ambilight (which I liked) of 32" that I moved to the bedroom
HD is coming in April, 4K will follow closely behind... the SCART sockets have become HDMI which are much more convenient... this might be the right time... or wait for the promotions that will surely come around May/June, for the football.