Its basic strength is 10¹⁰ more than l-dimethyl amino naphthalene.
Reason for high basic strength is :

Steric effects can indirectly affect acidity or basicity by affecting the resonance.
Steric effects can also be -caused by other types of strain. 1,8-bis(diethylamino)-2, 7-dimethoqmaphthalene (1) is an extremely strong base for a tertiary amine (pK_a of the conjugate acid = 16.3 ; compare N, N-dimethylaniline , pK_a = 5.1), but proton transfers to and from the nitrogen are exceptionaly slow ; enough to be followed by a uv-spectrophotometer. 1 is severely strained because the two nitrogen lone pairs are forced to be near each other. Protonation relieves the strain ; one lone pair is now  connected to a hydrogen, which forms a hydrogen bond to the other lone pair (shown in 2). The same effects are found in 4, 5-bis (dimethylamino) fluorene (3) and 4, 5 -bis (dimethylamino) .