Pull out lbeta from loop

This reimplements part of commit
e8421ab2a6dee75236b40bd9a4c72d36d8416a09.  See speed benefit

elapsed time: 23.780703054 seconds (4596333484 bytes allocated, 9.33% gc time)
elapsed time: 4.059966599 seconds (762523792 bytes allocated, 9.33% gc time)
elapsed time: 2.136129426 seconds (374194080 bytes allocated, 8.32% gc time)
elapsed time: 1.550982175 seconds (248907160 bytes allocated, 8.62% gc time)
elapsed time: 1.108897734 seconds (172428104 bytes allocated, 6.00% gc time)
elapsed time: 1.080372003 seconds (157042016 bytes allocated, 8.20% gc time)
elapsed time: 0.738366305 seconds (110850864 bytes allocated, 5.99% gc time)
elapsed time: 0.643303399 seconds (89060376 bytes allocated, 6.93% gc time)
elapsed time: 0.579980169 seconds (79410912 bytes allocated, 7.63% gc time)
elapsed time: 0.508957554 seconds (72065912 bytes allocated, 4.36% gc time)