-estimate on by default might cause slow compilation

[khanghugo]

From 12s down to 9s in this scenario for RAD step. YMMV but in TrenchBroom, it is as slow as 45s.

models            278/512        17792/32768    (54.3%)
planes           5760/32768     115200/655360   (17.6%)
vertexes        24155/65535     289860/786420   (36.9%)
nodes            9750/32767     234000/786408   (29.8%)
texinfos         4608/32767     184320/1310680  (14.1%)
faces           17167/65535     343340/1310700  (26.2%)
* worldfaces    13304/32768          0/0        (40.6%)
clipnodes       16374/32767     130992/262136   (50.0%)
leaves           6733/32760     188524/917280   (20.6%)
* worldleaves    4058/8192           0/0        (49.5%)
marksurfaces    22697/65535      45394/131070   (34.6%)
surfedges       80511/512000    322044/2048000  (15.7%)
edges           41935/256000    167740/1024000  (16.4%)
texdata          [variable]    9311108/33554432 (27.7%)
lightdata        [variable]     965775/50331648 ( 1.9%)
visdata          [variable]     272916/8388608  ( 3.3%)
entdata          [variable]     105937/2097152  ( 5.1%)
* AllocBlock       22/64             0/0        (34.4%)