I'm not really clear about whether your X5 is warming slowly, or overheating slowly.

If it's warming to operating temperature slowly, you can live with that. It may impact oil change intervals and/or your cats/

If it's overheating, you must fix ASAP. Especially if you have a 3.0. M54 blocks are often basically junk after they are overheated badly.

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2011 M3
2006 Sierra 2500HD 4WD LBZ/Allison
2004 X5 3.0i 6MT
1995 M3 S50B32
1990 325is
1989 M3 S54B32

Hers:
1989 325iX
1996 911 Turbo