Is it Delta-Delta or Delta-Alaska? If Delta-Alaska, it can take considerably longer at SEA than LAX (but still not bad; certainly under 20 minutes and probably under 15). If Delta-Delta, both LAX and SEA are easy. All Delta flights at SEA operate out of the S concourse; maximum 5 minute walk gate-to-gate (probably less). Even the T5-T6 walk at LAX (the worst case for a Delta-Delta connection) isn't bad, as others have said.

LAX is a zoo if you're doing a transfer that involves airlines other than Delta or Alaska, and security and check-in are often a zoo, but airside connections are not bad at all.